1. Field of the Invention
The invention relates to optical display systems, in particular to flat panel display systems containing ferroelectric material.
2. Statement of the Problem
One broad category of flat panel display systems comprises a luminescent, or phosphor, layer that is energized to produce visible light. A phosphor is a luminescent material that converts part of the absorbed primary energy into emitted luminescent radiation. (The term "phosphor", as used herein, includes any material that converts energy from an external excitation and, by means of the phenomenon of phosphorescence or fluorescence, converts such energy into visible light. The term "luminescent" as used herein includes "phosphor" as well as any other any other material or device which absorbs energy and thereby emits light.)
For example, in an electroluminescent (EL) display, an electric field is applied across the luminescent layer in sufficient magnitude to cause avalanche breakdown of the phosphor. The light generated by recombination of electron-hole pairs can be tuned in wavelength by the addition of various impurity ions to the phosphor. As in virtually all flat panel display (FPD) devices, the display panel is formatted in an X-Y matrix of pixels. The drive circuitry supports the application of individual voltage differences between two electrode layers at each pixel location. Unfortunately, the voltage required to trigger light emission from the luminescent layer in a thin-film EL device is as high as 200-250 V, and this requires that the driving circuits serving as switching elements should also be capable of withstanding such high voltage. The manufacture of such high-voltage devices is expensive. Furthermore, it is desirable that flat panel displays operate at the voltage level of many integrated circuit devices, that is, in the 3-10 volt range.
Flat panel field emission displays (FEDs) are also known. A field emission display typically comprises a flat vacuum cell with a matrix of microscopic field emitter cathode tips formed on the back plate of the cell, and a phosphor-coated anode at the front plate of the cell. The field emitter tips emit electrons upon application of appropriate voltages. The emitted electrons are directed to strike the luminescent layer with sufficient beam current intensity and kinetic energy to cause the luminescent layer to generate visible light.
An advantage of displays with phosphor layers is that backlighting of the display is thereby eliminated. Backlighting can be impractical because the color and intensity of the light is delivered to the display unmodified, and the system must modify it to produce an optical image. One typical way to include color in a backlighted display is to pass light through a color filter. But, the filter absorbs up to 70 percent of the incident light, resulting in inefficiency or low intensity. Similarly, methods forming an image by controlling the transmissivity of light through the panel also result in inefficiency. An advantage of FED systems, and phosphor-emission systems in general, is that the luminescent material generates the required image intensity based on the energy impinging the material without significant losses. Thus, displays with high brightness can be built. Unfortunately, FEDs typically require tens to hundreds of volts for electron emission, making it difficult to use these displays in many applications. Also, the electron field emitter tips typically need to be surrounded by a very high vacuum, at least 10.sup.-5 Torr, and often as high as 10.sup.-8 -10.sup.-9 in order to prevent degradation of the tips. Such high vacuums are difficult to maintain in the small volume enclosing field emitter tips. Furthermore, FEDs cannot be fabricated in "plane-to-plane" geometry.
It is known that ferroelectric materials can emit electrons when subjected to polarization switching. Ferroelectrics have the property of spontaneous polarization along a polarization axis. The material remains neutral internally as the end of each dipole is paired with the opposite end of the next dipole along that polar axis. At any boundary with a normal component to this axis, the dipoles are unpaired and a material-dependent bound charge will exist. As a consequence of this abnormally high energy state, free screening charges collect to neutralize the surface. It is possible to eject a pulse of these charges and/or induce a field emission pulse by altering the material's internal polarization. This process is not yet fully understood. The most common view of the process is that ferroelectric emission results from the expulsion of the free screening charge from the material's surface upon a rapidly induced change of the internal polarization. Another possibility is that ferroelectric emission is actually a field emission process wherein an extremely large electric field, generated by the spontaneous bound charge, is caused to exist across a nonferroelectric layer on the surface.
One advantage of a ferroelectric emission display, in particular, is that it can be fabricated in "plane-to-plane" geometry, which is not possible for field emission displays. Significant uses would include flat panel television screens and computer display devices.
Ferroelectric electron emission used in luminescent flat panel displays is known in the art. See, in particular, U.S. Pat. No. 5,453,661, issued Sep. 26, 1995 and U.S. Pat. No. 5,508,590, issued Apr. 16, 1996, which are hereby incorporated by reference as if fully contained herein. These disclose ferroelectric-emission FPDs. Both of these patents teach using lead zirconium titanate (PZT) and lead zirconium lanthanum titanate (PZLT) as ferroelectric electron emitters.
A second broad category of flat panel display system is the liquid crystal display (LCD). A liquid crystal layer in a flat panel display is arranged so that the molecules follow a specific alignment. This alignment can be changed with an external electric field, resulting in a corresponding change in the transmissivity of the liquid crystal material to light passing through it. Since the liquid crystal molecules respond to an external applied voltage, liquid crystals can be used as an optical switch, or light valve. In a typical configuration, the liquid crystal display comprises a front glass plate and a back glass plate. The space between the plates is filled with liquid crystal polymer. Various types of liquid crystal polymer are used. The principal classifications of liquid crystal material are twisted pneumatic, guest-host (or Heilmeier), phase change guest-host and double layer guest host. The type of liquid crystal employed determines the type of optical modulation that is effected by the light valve. For example, twisted pneumatic material reorients the polarization of the light (usually by ninety degrees). Guest-host materials, so-called by the presence of a dye that aligns itself with the liquid crystal molecules, modulate light as a consequence of the property of the dye to absorb or transmit light in response to the orientation of the liquid crystal molecules. In phase-change guest-host, the molecules of the liquid crystal material are arranged into a spiral form that blocks the majority of the light in the OFF state. The application of a voltage aligns the molecules and permits the passage of light. A double-layer guest-host liquid crystal comprises two guest-host liquid crystals arranged back-to-back with a ninety degree orientation between the molecular alignment of the two cells. Liquid crystal displays may be arranged to operate in a transmissive mode, requiring backlighting, or in a reflective mode for operation under high ambient light conditions, or in a combination of the two.
Liquid crystal displays are typically used such that pixels of liquid crystal material are arranged in a matrix form. The matrix displays are classified into passive and active types in terms of the driving method. In a typical passive display, transparent electrodes are patterned on both facing glass plates in perpendicular arrays. The repeating distance of the electrodes corresponds to the pixel dimension. In a typical active matrix, an active driving or switching device is provided for each pixel on a rear panel of the display. The driver is connected electrically to the edge of the display, and is switched with an external electrical signal. The conducting electrode is patterned to follow the pixel shape on the rear glass panel, but is a continuous film on the front plate.
Passive displays are easier to fabricate, but in practice are more difficult to operate. There are conducting lines on both sides of the display, and the drive circuits are more complicated. Passive displays use the multiplexing of signals on the opposing glass plates, which means that voltage pulses are repetitively intermixed and transmitted along row and column electrodes, combining at a cross point, that is, at the pixel being addressed. A pixel is turned ON when a voltage is present at both sides of the liquid crystal. One problem of a passive matrix is that a transparent conductor for both opposing plates must be patterned, and thousands of connections are required. Also, the response time of the more demanding liquid crystal material used in passive displays limits performance.
The limitations of a multiplexing scheme inherent in a passive display can be overcome by placing an active driving device behind each pixel. In an active display, the switch at each pixel simplifies the electronics of the display. The front panel is not patterned and simply acts as a ground electrode. Problems due to voltage nonuniformity are reduced or eliminated. Twisted pneumatic crystal material can be used instead of the more demanding supertwisted variety. The typical active matrix type liquid crystal display has a configuration in which memory elements each consisting of a capacitor and a nonlinear resistor element such as a diode or a transistor are connected to respective pixels. The capacitors are stored with charge while the nonlinear resistor elements are caused to operate in accordance with an input signal. The display continues to operate by virtue of the charge stored in the capacitors even after the input signal disappears, thus maintaining contrast in approximately the same level as that obtained by static driving (i.e., a static, constant signal).
The thin-film transistor is most commonly used as the active driving device, although the diode and MIM (metal-insulator-metal) element are also used in liquid crystal displays.
In an active matrix using thin-film transistors, image information (an input signal) is applied to the source electrode and transmitted to the liquid crystal, via an electrical channel that is on-off controlled by a voltage applied to the gate electrode, and stored as a charge by a capacitance of the liquid crystal. However, the charge held by the liquid crystal decreases with time because of leakage in each liquid crystal itself, a leakage current in the thin-film transistor, and other factors. Therefore, the contrast of a displayed image likely lowers with time. The complex process of forming the thin-film transistors and the resulting low yield make this type of matrix expensive to manufacture.
To solve the above problem, it is known in the art to use ferroelectric matrix drivers as the active driving devices. See U.S. Pat. No. 5,635,949 and U.S. Pat. No. 4,021,798, which are hereby incorporated by reference as if fully contained herein. A ferroelectric element thereby replaces transistors, diodes, and nonlinear MIM elements. With a ferroelectric material, it is possible to produce high quality images by maintaining the charge in the liquid crystal material with a relatively simple structure and a reduced number of production steps.
An active ferroelectric driving device of a liquid crystal display pixel utilizes the ferroelectric's remnant polarization, in which even after application of an electric field to the ferroelectric material has ceased, an electric field caused by remnant polarization remains in the material. The remnant polarization is decreased, eliminated or reversed by applying an electric field of opposite polarity. After a voltage has been applied to the ferroelectric material portion of an active switching element, an internal electric field remains in the ferroelectric material due to the remnant polarization. The internal electric field causes a remnant voltage to be applied to the liquid crystal portion of the display pixel. The driver can be designed so that the remnant voltage across the liquid crystal portion is large enough to selectively influence the transmittance of light through the liquid crystal portion. As a result, it becomes possible to provide a liquid crystal display capable of producing clear, high-contrast images. However, the ferroelectric portion in such a display must possess high residual polarizabilty in order to maintain a large remnant electric field in the liquid crystal portion. Also, the ferroelectric material should possess very low leakage characteristics, so that the remnant electric field does not dissipate rapidly.
In both known applications of ferroelectric material in flat panel displays, that is, as an electron emitter and as an active-matrix driving element in a LCD, the ferroelectric properties are used to transfer energy from the ferroelectric portion to a nonferroelectric portion of the flat panel display. In both applications, the transfer of energy and the overall function of the ferroelectric portion depends ultimately on polarizabilty and polarization-switching in the ferroelectric portion. In addition, to operate a typical flat panel display, the driving system scans each pixel 100-300 times per second. In the art, it has been suggested to use ceramic ferroelectric oxides, namely lead zirconium titanate (PZT) and lead lanthanum zirconium titanate (PLZT), as the ferroelectric element in both electron emitters and active matrix switching devices in LCDs. Both PZT and PLZT possess high polarizabilty relative to other ferroelectric materials. For example, when subjected to a saturating electric field, PZT capacitors with a thickness in excess of 300 nm typically show remnant polarization values, 2Pr, of about 35 .mu.C/cm.sup.2 (e.g., see U.S. Pat. No. 5,519,234, FIG. 25). In the study reported by Auciello et al., Appl. Phys. Lett. 66 (17), 2183, the 2Pr-value of PZT-capacitors with a thickness of 800 nm was measured to be 40-50 .mu.C/cm.sup.2. Also, both PZT and PLZT can be switched rapidly, on the order of tens of nanoseconds. On the other hand, the polarizabilty of PZT and PLZT drops precipitously as film thickness decreases below 300 nm. Below 100 nm, the 2Pr-value of PZT approaches zero. Also, PZT and PZLT show fatigue symptoms immediately upon being subjected to voltage switching tests. Fatigue means a deterioration of desired ferroelectric properties as a result of polarization switching. The 2Pr-value of PZT and PZLT can drop to one-half its initial value after about 10.sup.6 polarization switching cycles. PZT and PZLT thin films also typically show a high leakage current of about 10.sup.-6 A/cm.sup.2.
It is, therefore, desirable to find structures of flat panel displays and methods of fabricating and using such structures that improve those already known in the art. In particular, it is desirable to find a material to use in flat panel displays, either as an electron emitter or as part of the active driving element of a liquid crystal portion, that possesses manufacturing or operating characteristics that are superior to those of PZT, PZLT, and other ferroelectric compounds known in the art. It is also desirable to find improved driving elements for the pixel elements in flat panel displays.
3. Solution to the Problem
It is an object of this invention to provide ferroelectric optical display systems, in particular flat panel display systems containing a ferroelectric layered superlattice material.
A feature of the invention is the use of ferroelectric layered superlattice materials in an optical display device to selectively influence the operation of an optical element of the device. The invention relates particularly to flat panel displays useful as viewing screens in devices such as computers and televisions.
Another feature of the invention is that the layered superlattice material can be deposited as a thin film with a thickness in the range 5-400 nm, preferably in the range 50-140 nm, and most preferably with a thickness of about 100 nm.
In one embodiment of the invention, the optical display contains luminescent material, and the layered superlattice material is caused to emit electrons that impinge the luminescent material to cause it to emit light.
In another embodiment of the invention, the optical display contains liquid crystal material, and the ferroelectric layered superlattice material is polarized to exert an electric field in the liquid crystal material, thereby selectively influencing the transmissivity of light through the liquid crystal material.
One aspect of the invention is the use of precursors that contain metal moieties in effective amounts for spontaneously forming in optical displays a ferroelectric layered superlattice material upon drying and heating of the precursor. The precursors preferably contain a polyoxyalkylated metal portion having a molecular structure including a metal-oxygen-metal bond.
Another feature of the invention is that the layered superlattice material can contain amounts of the so-called superlattice generator elements and B-site elements in excess of the stoichiometrically balanced amounts. Excess amounts of such elements enhance certain desired properties of the layered superlattice materials, such as low imprint and low fatigue.
In preferred embodiments of the invention, the layered superlattice material comprises strontium bismuth tantalate, and at least one of the metals bismuth and tantalum is present in an excess amount.
In other preferred embodiments of the invention, the layered superlattice material comprises strontium bismuth tantalum niobate, and at least one of the metals bismuth, tantalum and niobium is present in an excess amount.
Another aspect of the invention is a method for fabricating a ferroelectric device in an optical display. The method generally includes providing a substrate; providing a precursor containing metal moieties for spontaneously forming a ferroelectric layered superlattice material upon drying and heating the precursor; applying the precursor to the substrate; drying the precursor to form a dried material on said substrate; and heating the dried material at a temperature of between 500.degree. C. and 1000.degree. C. to yield a layered superlattice material containing the metals. Preferred embodiments of the precursor contain an excess amount of at least one of the superlattice generator and B-site elements. Other preferred embodiments of the precursor contain metal moieties in effective amounts for forming strontium bismuth tantalate or strontium bismuth tantalum niobate. Preferred embodiments of such precursors also contain excess amounts of at least one of bismuth, tantalum and niobium.
In a preferred embodiment of the invention, an optical display contains a thin film of a ferroelectric functional gradient material ("FGM"), or functionally graded material. In one basic variation, a FGM thin film that serves as an electron emitter contains a ferroelectric compound and a dielectric compound, wherein the dielectric compound has a dielectric constant less than the dielectric constant of the ferroelectric compound. The ferroelectric FGM thin film is characterized by a molar concentration gradient of the ferroelectric compound between regions of the FGM thin film. The concentration gradient may be gradual or it may be stepwise. Typically, there is also a concentration gradient of the dielectric compound in the ferroelectric FGM thin film, usually in a sense opposite to the direction of the gradient of the ferroelectric compound. The ferroelectric FGM is oriented such that the direction of the concentration gradient of the ferroelectric compound is positive in the direction of electron emission and the polarizabilty of the FGM thin film is highest near the emission surface. As a result of the functional gradient, the electron density at the emission surface of the ferroelectric FGM thin film is higher than if no dielectric compound were present. Therefore, for a given electric field across the ferroelectric FGM thin film, the energy intensity of the emitted electrons is correspondingly greater.
In a second basic variation, the FGM thin film is a functional gradient ferroelectric ("FGF"), or functionally graded ferroelectric, thin film. In a FGF thin film, the concentration of a plurality of ferroelectric compounds varies across the thin film. Typically, the molar concentration of a plurality of ferroelectric compounds in a class of compounds having similar crystal structures is varied across the FGF thin film. The changing concentration of different compounds is a result of a change in the relative amounts of one or more types of metals across the thin film. For example, a FGF thin film may contain the metal types strontium, bismuth, tantalum and niobium in relative molar proportions corresponding to a generalized stoichiometric formula SrBi.sub.2 (Ta.sub.1-x Nb.sub.x).sub.2 O.sub.9, where x may vary in a range of 0.ltoreq.x.ltoreq.1. The generalized stoichiometric formula represents a class of ferroelectric layered superlattice material compounds with similar crystal structures. A concentration gradient of tantalum and niobium corresponding to changes in the value of x represents a functional gradient of the ferroelectric compounds. The term "type of metal" and similar terms refer to a type of atom corresponding to a chemical element from the periodic table of chemical elements. For example, titanium, zirconium, tantalum, niobium and lanthanum are five different types of metal. In an optical display according to the invention in which the FGM thin film is a FGF thin film, the polarizabilty varies corresponding to the gradient. The FGF thin film is oriented such that the maximum polarizabilty is at the surface from which electrons are emitted.
In embodiments of the invention containing the novel feature of a ferroelectric FGM thin film, the ferroelectric compounds may be selected from a group of suitable ferroelectric materials, including but not limited to: ABO.sub.3 -type metal oxide perovskites, such as a titanate (e.g., BaTiO.sub.3, SrTiO.sub.3, PbTiO.sub.3, PbZrTiO.sub.3) or a niobate (e.g., KNbO3), and, preferably, layered superlattice compounds.
A method of the invention for fabricating a FGM thin film includes applying sequentially a plurality of precursor solutions to a substrate to form a functional gradient. The relative concentrations of types of metals in the precursor solutions varies, corresponding to the functional gradient desired.
According to the invention, the ferroelectric FGM thin film may be applied using any number of techniques for applying thin films in integrated circuits. Preferably, metal organic precursors suitable for metal organic decomposition ("MOD") techniques of thin film deposition are used. MOD methods enable convenient and accurate control of precursor concentrations. Preferably, a multisource chemical vapor deposition ("CVD") method is used. In the preferred method of the invention, the mass flow rates of individual precursor streams into the final precursor mixture applied to the substrate are individually and accurately varied during the course of the deposition process to form the inventive functional gradients in the ferroelectric FGM thin film.
An important feature of the invention is the novel use of a varistor device in an optical display. The nonohmic current flow through the varistor device selectively modifies the voltage drop across a ferroelectric thin film, depending on the voltage applied to the varistor. Here, the word "modify" means that the voltage input to the varistor is not the same as the voltage output by the varistor. Voltage across the ferroelectric thin film determines the electric field across the ferroelectric thin film and, therefore, polarization switching behavior. At relatively low voltages, the resistance across the varistor is relatively high. As a result, at low voltages, the electric field across the ferroelectric thin film is disproportionately small. As voltage amplitude from a variable voltage source increases, however, the resistance of the varistor decreases, and the voltage drop across the ferroelectric thin film increases nonlinearly. The result is a relatively sudden and sharp increase in the electric field. The varistor, thereby, allows a display pixel to suppress "cross-talk" from a neighboring pixel when the neighboring display pixel is addressed by voltage signals. The inventive varistor also enables a sharper, more sudden reversal of voltage bias and, therefore, polarization across a ferroelectric thin film serving as an electron emitter. As polarization switching becomes more sudden, the surface electrons on a ferroelectric thin film have less time to adjust to the change in polarization and are emitted with greater energy intensity. This use of a varistor device should not be confused with the use of diodes and nonlinear resistance devices instead of ferroelectric elements in LCDs of the prior art.
A further feature of the invention is a structure in which a plurality of ferroelectric thin films serve as electron emitters in a display pixel. Typically the ferroelectric thin films are at opposing, parallel sides of a phosphor layer. Such a structure is suitable for the application of alternating current voltage sources to cause electron emission during each phase of the voltage cycle. In another embodiment, a ferroelectric thin film electron emitter is located on one side of a phosphor layer, and a dielectric thin film is located at the opposing side. Application of a low switching voltage to an electrode for the ferroelectric thin film causes electron emission. Application of a high alternating current voltage to an electrode at the dielectric layer causes thin film electron luminescence ("TFEL").